Ventilating buildings, particularly office buildings, by blowing air into them is known. Air entering through an intake on the building's roof, for example, so that it is as clean as possible, is blown in by a fan under high pressure inside the main duct to which are connected branch ducts going to the various rooms, the branch ducts being equipped with balancing registers that may or may not be regulatable. However, balancing is never perfect. Moreover, these systems have a regulating system in each ventilated room. Thus there must be a register in each room to perform this regulation. This register produces noise problems, however, because the register is close to the room or zone into which the air is diffused. Hence it is necessary to provide a sound trap such as a grid with a large cross section to cover the whole. This structure takes up a great deal of space and some of the component parts such as the register are hidden, which makes for difficult maintenance.